1. Field of the Invention
The present invention relates generally to thermal control of opto-electronic devices and particularly to the control of optical properties of an optical fiber device by thermal manipulation.
2. Technical Background
Many optical devices have optical properties that change with changes in temperature. Examples of these devices are fiber Bragg gratings (FBGs), long period gratings (LPGs) and Mach-Zehnder devices. These devices are used in many optical applications; in particular, fiber Bragg gratings are used in wavelength add-drop multiplexers where one particular wavelength is added or dropped from a series of multiplexed wavelength signals transmitted in a single optical waveguide fiber. Long period gratings and tunable long period gratings are used in gain flattening and gain equalization applications. The center wavelength of a grating may be changed by subjecting the grating to mechanical strain or operating the grating at different temperatures.
One approach utilizing thermo-optic properties to tune an optical device requires active monitoring of the optical signal being processed by the device. Traditionally this is accomplished using an optical feedback control system.
The present invention relates to athermalizing optical elements and tuning one or more of the optical properties of the optical element by controlling the heat applied to the optical elements.
One aspect of the present invention is a tunable optical device that includes an optical fiber device having optical properties that vary with temperature, and a resistive heater. The resistive heater includes a metal layer that is thermally coupled to the optical fiber device and two electrical contacts that are electrically connected to the metal layer. The electrical contacts are spaced apart from one another along the metal layer, and the portion of the metal layer between the contacts is the region that serves as the resistive heater. The invention also includes a controller that is electrically connected to the contacts. The controller both provides electrical power to the heater and measures the electrical voltage across the heater. The controller compares the measured voltage across the heater to a pre-selected reference value. The controller then regulates the amount of electrical power supplied to the heater. By regulating the amount of electrical current supplied to the heater the temperature of the heater is controlled.
One advantage of the present invention over prior methods of controlling the optical properties of an optical fiber device by thermal means is that the present invention incorporates the temperature sensor and heater element into one element. This is accomplished by using a resistive heater having a temperature dependent electrical resistance and measuring the resistance of the heating element. Traditionally, separate mechanisms for measuring the temperature, such as thermocouples, are used. The use of a separate heater and sensor requires a more complex control circuit and device packaging.
Another advantage of the present invention is that when the combination heater/sensing element is a metallized tube, the assembly process is greatly facilitated. The improvements in the assembly and fabrication processes are due in large part to not having to metallize fiber gratings, thus, the difficulty in metallizing a fiber is avoided. Furthermore, the system is usable with long period gratings. For optical property reasons tunable long period gratings must be surrounded with a material of a specific index of refraction, such as a sol gel material. The slotted tubular heater when used for long period grating packaging provides a reservoir area for putting the sol gel or polymer material in, until it can be thermally cured around long-period grating.
Another advantage of the present invention is realized by setting the temperature of the optical fiber device at a fixed level above the environmental operating range of the module into which it is to be installed. This form of packaging achieves an effective, low cost, and simple active athermalization of the device; this ensures stability of the desired optical properties throughout the entire environmental operating range specified for the module in which the device is installed.
An advantage of the invention is that it has compact size, which is increasingly important as the trend in optical systems is to require greater numbers of optical components fit in a given space.
Yet another advantage of the invention is that can be used as a temperature controlled tension/compression tuned fiber Bragg grating. The tubular heater can be put on a grating section and can hold the grating at a constant temperature that is slightly above the maximum operating temperature. The center wavelength of the fiber Bragg grating is now determined solely by the tension/compression applied to the grating. A low glass transition temperature material is preferred as the filling around the fiber Bragg grating, so the tension/compression on the grating does not cause any stress on the metal coating that is on the inner wall of the slotted tubular heating sensing element.
Yet another advantage of the present invention is that it may be used to make a thermally chirped fiber Bragg grating. By applying a temperature gradient along a fiber Bragg grating length, the fiber Bragg grating can be thermally chirped to change the grating bandwidth and dispersion, which are useful in wavelength add-drop multiplexing and dispersion management applications. A thermal gradient can be generated using the tubular heater embodiment of the present invention. A number of heaters, separated by distances that depend on the chirping effects in a thermal gradient that is to be established, are placed along the length of the grating and these heaters are then operated at different temperatures. Alternatively, a temperature gradient may be generated by modifying the effective resistance per unit length of the metal coating of a single tubular heating element. This may be accomplished by laser ablation of the metal layer.
Another advantage of the invention is that it can be used to produce a dual heater thermally chirped fiber Bragg grating. This is accomplished by using two independent heating elements that are coaxially arranged around a fiber grating. A segmented heater as described above may be used to generate a degree of chirp, and a second segmented or uniform heater can be used to set the bias point of the temperature of the grating. In this manner, chirp and center wavelength can be controlled independently. Another advantage of this configuration is that control over the spectral and dispersive properties of the fiber grating is facilitated.
Another advantage of the present invention is that it allows a fiber optical device to be calibrated and assembled with its controller into a single discrete package.
Another advantage of the present invention is that the calibration of each optical device during the assembly process greatly facilitates adjusting the center wavelength of the device.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.